05apr'06: I reran a set of 416 MIS visits allowing match pairs with separations up to 7 arcseconds and any probability. I restricted the set to only include visits with at least 1000 seconds.
From these data, it is
reasonable to set the probability limit at 10^-5 and separation
limit at 5 arcseconds:

galexmerge ... -prob 0.00001 -sep 5.0

Assuming TOT_IBF_ERR=0.53, here is
a distribution of probabilities: prob2_1.ps .
And at the low end: prob2_2.ps .
We can see how log of the probability depends on separation for
bright, compact sources: sep_prob_1.ps .
There is a much wider distribution if we include faint sources:
sep_prob_2.ps . And if we allow sources
with wide FWHM we get: sep_prob_3.ps .
Even in this case, the number of source pairs allowed (prob > 10^-5) between 4 and 5 arcseconds
is relatively small ( 1.5% ), and
the number of source pairs not allowed with prob>10^-5 and sep>5. is 0.15%.
After fixing the probability computation bug (nu=1 instead of nu=2) and
correctly computing the x,y positional errors (straight average, not quadrature),
I compute new values of:

TOT_IBF_ERR N_FLDERR F_FLDERR
New 0.53 0.43 0.69
Old 0.51 0.42 0.68

Here is a plot of sigmas compared with a gaussian distribution for matched pairs (NUV,FUV):
New_ibf_sigmas.ps .
Here is a plot of Chi-Squared probabilities for matched pairs (NUV,FUV):
New_ibf_prob.ps .
The excess of small probability values indicates either false matches or
non-gaussian wings to the error distribution:
New_ibf_prob_zoom.ps .
Here is how probability varies with separation:
New_Sep_Prob.ps .

This analysis was done on the latest MIS imaging data ( 90 visits in
/home/runes11/tim/work/cal3bigrun/01-vsn/*/d/00-visits/*-img/02-try/ )
with the most current NLC and flat field.

We need to compute three parameters for the bandmerger:
The interband field error (IBF_ERR = TOT_IBF_ERR/sqrt(2)),
the NUV field error (N_FLDERR), and the FUV field error (F_FLDERR).
The total radial interband position error is given by:

These last two are the position errors relative to an absolute (true)
reference frame. SDSS source positions are used to provide the
true frame.

To compute the TOT_IBF_ERR, values are tried and a comparison is done between the
actual separations divided by the computed total radial position error (sigmas)
and a gaussian distribution.
Here is a comparison of a few different values:
new-compare.ps .
The best solution is TOT_IBF_ERR=0.51,
see: new-tot_ibf_err.ps .
(This implies IBF_ERR=0.36 arcseconds.)
For this analysis, an abridged list was used:
n/fuv_mag >16, <21, n/fuv_fwhm_image <10 arcseconds, and fov_radius < 0.55 degrees.